Changes

|Feature overview=Add-ons are synchronized between sync Sync clients. Sync will ensure that add-ons are installed, uninstalled, updated, and enabled as they are changed on each client.|Feature users and use cases=; Install propagation : User installs an add-on on one browser. When a sync occurs, this add-on record is uploaded to the Sync server. When another browser in the Sync profile connects, the add-on is automagically installed automatically. ; Uninstall propagation : This is the same as install propagation except for uninstalls. An uninstall on one client will trigger an uninstall on another. ; User enabled state propagation : User opens add-on manager and disables an add-on. Sync will propagate this change to other clients. Ditto for enabling. Sync should not propagate the enabled state if it is due to application compatibility (e.g. version upgrades causing incompatibilities). Only the user's explicit enabling and disabling of an add-on should be propagated. ; Update propagation : User updates an add-on on one machine. Sync notices the update and schedules other sync clientsto look for an update.|Feature dependencies=; Sync GUID added to XPI Provider : Sync requires non-deterministic IDs to be affiliated with synchronized records. XPI add-ons currently don't have a suitable ID so once will need to be created.|Feature requirements=; Non-impact on add-on manager metrics : The presence of Sync should not skew the metrics in the add-on manager and addons.mozilla.org. Currently, some APIs on the AddonManager upload metrics.|Feature non-goals=Sync will not synchronize non-XPI add-ons such as plugins, lightweight themes, and search engines. Sync will also not synchronize add-ons installed outside of the currently running profile.